This invention relates to air conditioning systems, and more particularly to a compression type mechanical refrigeration air conditioning system for installation in a high voltage, direct current driven railway vehicle.
In recent years it has become almost universal practice to provide compression type mechanical refrigeration air conditioning systems in all types of passenger railway vehicles. Normally, reciprocating type compressors have been used in these air conditioning systems and they have been driven by a motor having an on-off control. Such on-off systems have not been satisfactory because the cooling system capacity cannot be varied in a manner such as to meet the widely varying cooling needs of the vehicle and passengers. Also, the cycling of compressors had been an inefficient manner of operation and subjects the valves and other components in its system to undue stress and unnecessary wear.
Another problem in such systems is the motor used to drive the compressor. Initially, direct current motors were used because, in the case of rapid transit and other electrically driven vehicles, the only practical power source for the compressors is the high voltage DC source used to power the vehicle, which may be picked up by means of a third rail or by means of an overhead wire and pantograph. This power source has not been entirely satisfactory because of the severe transients which are often developed as the vehicle passes over switches and open blocks where power may be temporarily interrupted. Furthermore, the power supply voltage, which typically measures in excess of 600 volts and may vary widely over different section of track, is difficult to regulate and presents special motor insulation and stability problems. Attempts at reducing the voltage applied to the DC motors by provision of series resistors between the power source and the motors have not only undesirably wasted power, but have also created additional cooling and ventilation problems for railway vehicle manufacturers.
Another problem with prior art DC compressor motors is that they require periodic brush replacement and frequent overhaul of commutator segments due to arcing across the segments. The high maintenance required of these motors makes their use in railway vehicles unnecessarily costly and detracts from the reliability of the vehicles, which often operate in environments where an operative air conditioning system is necessary for the vehicles to remain in service.
One potentially successful solution to this problem has been to use AC induction motors in the air conditioning system, in conjunction with an onboard means to convert the third rail direct current to alternating current. Induction motors because they have no commutator and brushes are substantially more reliable than DC motors. However, previous approaches to providing such onboard power converters have not been entirely satisfactory. For instance, one approach has been to provide a motor-alternator set on the railway vehicle to convert the third rail direct current to alternating current. Difficulties have been encountered due to the weight and complexity of motor-alternator sets which add unnecessarily to the vehicle's gross weight and hence increased cost of manufacture and operation of the vehicle. Further, motor-alternator sets themselves require periodic brush replacement and maintenance and therefore at lest achieve only a partial reduction in maintenance. Prior art attempts at providing static solid state, DC to AC inverters for this purpose have also been unsuccessful because the adverse electrical and physical environment in which such inverters must operate has heretofore prevented the degree of stability necessary for successful application to railway vehicles air conditioning systems.
An attempt has also been made to use in such an air conditioning system a helical screw type compressor driven by and AC induction motor. A DC to AC inverter supplied power to the motor and the compressor was operated continuously during the cooling mode of operation. An on-off thermostatic controlled refrigerant bypass valve was used to control temperature. Such a system, however, did not operate properly and was abandoned.
Accordingly, an object of the present invention is the provision of a new and improved compressor type, mechanical refrigeration system for a railway vehicle operable from a high voltage DC power line and providing continuous duty cooling at a rate dependent on the cooling demand of the vehicle interior.
Another object of the present invention is the provision of a compressor type, mechanical refrigerant air conditioning system for a vehicle utilizing an induction motor to drive a compressor at speed dependent on the cooling demand of the vehicle's interior.